Various type of non-hydrocarbon compounds occur in crude oil ad refinery streams. The most important are the organic sulfur , nitrogen and oxygen compounds. Trace of metallic compounds are also found in all crudes. The presence of these impurities is harmful and may cause problems to certain catalytic processes. Fuels having high sulfur an nitrogen levels cause pollution problems in addition to the corrosive nature of their oxidation products.
Sulfur in crude oil is mainly present in the form of organosulfur compounds. Hydrogen sulfur is the only important inorganic sulfur compound find in crude oil. Its presence however, is harmful because of its corrosive nature organosulfur compounds may generally be classified as acidic and non acidic. Acidic sulfur compounds are the thiols (mercaptans). Thiophene, sulfides and disulfides are examples of non-acidic sulfur compounds found in crude oil fractions.
Extensive research has been carried out to identify some sulfur compounds in a narrow light petroleum fraction.
Examples of some sulfur compounds from the two types:
Acidic sulfur compounds:
Non-acidic sulfur compounds:
Sour crude contain a high percentage of hydrogen sulfide. Because many organic sulfur compounds are not thermally stable, hydrogen sulfide is often produced during crude processing. High-sulfur crude are less desirable because treating the different refinery streams for acidic.
Hydrogen sulfide increases production costs. Most sulfur compounds can be removed from petroleum streams through hydrotreatment processes, where hydrogen sulfide is produced and the corresponding hydrocarbon released. Hydrogen sulfide is then absorbed in a suitable absorbent and recovered as sulfur.